[PATCH v2 0/3] no_profile fn attr and Kconfig for GCOV+PGO

[Nick Desaulniers]

The kernel has been using noinstr for correctness to politely request
that the compiler avoid adding various forms of instrumentation to
certain functions.

GCOV and PGO can both instrument functions, yet the function attribute
to disable such instrumentation (no_profile_instrument_function) was not
being used to suppress such implementation. Also, clang only just
recently gained support for no_profile_instrument_function. GCC has
supported that since 7.1+.

Add a new function annotation __no_profile that expands to
__attribute__((__no_profile_instrument_function__)) and Kconfig values
CC_HAS_NO_PROFILE_FN_ATTR and ARCH_WANTS_NO_INSTR. Make GCOV and PGO
depend on either !ARCH_WANTS_NO_INSTR or CC_HAS_NO_PROFILE_FN_ATTR.

Changes V1 -> V2:
* s/no_profile/no_profile_instrument_function/
* fix trailing double underscore on GCC 4 define, as per Fangrui+Miguel.
* Pick up Fangrui + Miguel's reviewed-by tag.
* Add link to GCC's doc.
* Fix clang's doc format; will appear once clang-13 is released.
* New cleanup patch 2/3. Orthogonal to the series, but while I'm here...

Base is
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=for-next/clang/pgo.

Nick Desaulniers (3):
  compiler_attributes.h: define __no_profile, add to noinstr
  compiler_attributes.h: cleanups for GCC 4.9+
  Kconfig: add ARCH_WANTS_NO_INSTR+CC_HAS_NO_PROFILE_FN_ATTR, depend on
    for GCOV and PGO

 arch/Kconfig                        |  7 +++++++
 arch/arm64/Kconfig                  |  1 +
 arch/s390/Kconfig                   |  1 +
 arch/x86/Kconfig                    |  1 +
 include/linux/compiler_attributes.h | 19 ++++++++++++++++---
 include/linux/compiler_types.h      |  2 +-
 init/Kconfig                        |  3 +++
 kernel/gcov/Kconfig                 |  1 +
 kernel/pgo/Kconfig                  |  3 ++-
 9 files changed, 33 insertions(+), 5 deletions(-)


base-commit: 4356bc4c0425c81e204f561acf4dd0095544a6cb
-- 
2.32.0.288.g62a8d224e6-goog